U.S. auto safety regulators have stepped up a series of investigations into engine fires that have plagued Hyundai and Kia vehicles for more than six years.

Engine failures and fires have dogged the Korean automakers’ vehicles since September 2015 when the company issued an engine failure recall. Since then it has issued at least eight more recalls for a host of engine problems, according to NHTSA documents posted on its website Monday.

The agency says it’s opening the engineering analysis to evaluate whether previous recalls covered enough vehicles. It also will monitor the effectiveness of previous recalls “as well as the long-term viability of related programs and non-safety field actions being conducted by Hyundai and Kia.”

The agency says it’s opening the engineering analysis to evaluate whether previous recalls covered enough vehicles. It also will monitor the effectiveness of previous recalls “as well as the long-term viability of related programs and non-safety field actions being conducted by Hyundai and Kia.”

The engineering analysis could lead to further recalls.

Hyundai said on Monday that it is cooperating fully with U.S. regulators.